Oxidation-resistant graphite article and method



United States Patent ()fitice Fates-lied Nov. 20, 1962 3,065,088@XEDA'llON-RESESTANT GRAPHITE ARTICLE AND METHOD Milton Janes, Lakewood,and Sheldon A. Taylor, Berea,

Ohio, assignors to Union Carbide Corperation, a corporation of New YorkNo Drawing. Filed Sept. 30, 1959, Ser. No. 843,364 5 Claims. (Cl. 10644)This invention relates to oxidation-resistant articles and it moreparticularly refers to such articles which are basically graphitic innature.

Recent developments in the field of aerodynamics have revealed a needfor materials which will endure under conditions of high heat flux, withconsequent high surface temperatures, and high velocity oxidizing gasflow relative to the material. It has been discovered that such amaterial must also be resistant to thermal shock since the conditionsencountered lead to very rapid temperature rise. Graphite is one of thebest refractory materials known today which is adapted to fulfill therequirements above set forth. However, graphite suffers from the faultof being subject to rapid oxidation and consequent erosion under theaction of high velocity oxidizing gas streams.

It is therefore the principal object of this invention to providegraphitic articles which are relatively oxidation resistant.

It is another object of this invention to provide a method of producingsuch articles.

Fulfilling these objects this invention broadly comprises anoxidation-resistant article comprising graphite having silicon carbideand either titanium carbide, boron carbide or mixtures thereofsubstantially uniformly distributed throughout said article.

An article according to this invention may be produced by mixing someform of comminuted carbon, such as petroleum coke for example, with acarbonizable binder, suitably coal tar pitch, and either boron carbide,titanium carbide or mixtures thereof. After thorough mixing, thesematerials are formed into the desired shape by molding, extruding orother suitable means, and then the formed article is packed in aprotective covering, preferably coke, and baked to approximately 850 C.according to the usual practice in the carbon industry. This bakingcarbonizes the binder thus curing the article and making itdimensionally stable. After the article is cured, the protectivecovering is removed and replaced with a siliconcontaining packing. Thethus-packed article is placed in a suitable graphitizing furnace andheated to a graphitizing temperature, suitably higher than 25 C., in aconventional manner. After a suitable time at the graphitizingtemperature, which time depends upon the size of the article beingproduced, the graphitized article is cooled and removed from thefurnace. The article thus produced is basically graphitic in naturehaving either boron carbide, titanium carbide, or mixtures thereof andsilicon carbide substantially uniformly distributed throughout thearticle.

The article according to this invention initially should suitablycontain to 50 weight percent boron carbide, titanium carbide or mixturesthereof and should preferably contain 15 to 35 weight percent of suchcarbide after the initial baking step. A thus-constituted article, whenproperly graphitized according to the method described above, will pickup between and 70 percent silicon from the graphitization packing. Thesilicon picked up reacts to form silicon carbide inside the graphitizedarticle. The amount of silicon pick-up and the amount of other carbidepresent in the article is determinative of the oxidation resistance ofthe finished graphitized article. The proportions of each constituent inthe finally graphitized article based upon the total weight of the finalarticle are about 30 to weight percent graphite, about 15 to 60 weightpercent silicon carhide, and about 10 to 30' weight percent in theaggregate of boron carbide, titanium carbide or mixtures thereof. It ispreferred to have a silicon carbide concentration of from 30 to 50weight percent.

It has been determined that the amount of silicon picked up to formsilicon carbide from the silicon-containing packing during thegraphitization step described above is a function of the graphitizingtime and temperature and is also proportional to the amount and natureof the other carbides introduced into the mix from which the article ismade. Table I below is a compilation of data comparing the silicon pickup for articles /2 inch by /2 inch by 4% inches containing 18 weightpercent boron carbide which were graphitized at temperatures from 2500C. to 2900 C. for 30 minutes.

Table II below is a compilation of data comparing the silicon pick up,for articles /2 inch by /2 inch by 4% inches which have been graphitizedat 2700 C. to 2800" C. for 30 minutes, to the amount of boron ortitanium carbide initially added to the mix from which the article wasmade.

Table II Boron carbide in original mix (percent) 7 11 17 29 0 14Titanium carbide nal mix (percent) 0 0 0 0 29 14 Silicon weight gain(Dercent of sample weight850 C. bake) 30. 2 36.1 40. 5 46. 5 12. 5 47. 3

From a consideration of Table I and II, it may be seen that whilesilicon vapor is picked up from the packing during graphitization at anytemperature up to 2900 C., an optimum graphitization temperature is2700" C. to 2800 C. Similarly, the incorporation of amounts of boron ortitanium carbide up to 50 Weight percent induce silicon pick-up from thepacking. However the best results are obtained when 15 to 35 weightpercent boron or titanium carbide or mixtures thereof is present in thebody which is baked to 850 C.

Many articles have been made according to the practice of this inventionand tested to determine the amount of resistance to oxidation impartedby the method disclosed herein. This testing consisted of resistanceheating a test article to a given temperature by passing an electriccurrent therethrough and directing a stream of air, initially at roomtemperature, at the heated article. Table III below is a compilation ofdata taken from the various tests run. The measure of oxidationresistance in these tests is the time it takes for a sample article toburn through thereby interrupting the current flow. Each of the articlestested was Mr inch in diameter and 4 inches long.

aeeacas 3 Table III Composition of Article Final Composition baked to850 C. Graphiti- Percent Failure zation GraphitiZawt. gain Test Temp.time Tempera tion packing from (minutes) Percent Percent Percent ture,C. packing Percent Percent Percent Percent B40 TiO 0 B40 TiG S10 M 1 1002, 800 coke 0 100 4,800 Watts 099.

input. 2 81.9 18.1 2, 500 d0 0 84. S 14.6 0.6 do 0.86. 3 81.9 18.1 2,500 SiO+coke 7. 37 76.1 13. 10.4 d0 0.92. 4 81.9 18.1 2, 800 do 3S. 240.6 10. 0 39.8 do 15.9. 5 81.9 18.1 2, 800 coke 0 84. 8 14. 6 0.6 3,600watts 0.83.

' input. 81. 9 18.1 2,700 SiO+c0ke 52. 0 41. 2 9.6 341), 81.9 18.1 2800.do 45.0 45.2 10.1 88.3 12.7 1, 2, 900 do 22. 7 65. 8 7. 4 81. 9 18.1 2,725 do 47.1 43. 0 10.0 60. 4 30.6 2, 700 do 40. 5 36.3 18. 4

30. 6 2, 800 coke 0 30. 6 2,800 SiC+eoke 26. b 30.6 2,700 do 14.0 15. 32, 800 coke 0 15. 3 2,800 SiO-l-coke 58.0

An analysis of the data presented in this table reveals that theoxidation resistance of an article which is basically graphitic incomposition is increased provided the proper carbide in the properproportions is incorporated into the mix from which the article isformed and that such formed, cured article is graphitized at the propertemperature while it is protected by a silicon-containing pack. Acomparison of runs 5 vs. 7, 6 vs. 7, 7 vs. 8, and 10 vs. 13 points upthe value of having all the variables noted above at their optimum inorder to insure oxidation resistance.

What is claimed is:

l. A formed graphite oxidation-resistant article having silicon carbideand at least one member of the group con sisting of boron carbide andtitanium carbide substantially uniformly distributed therethrough.

2. A formed oxidation-resistant article consisting essentially of 10 to30 percent by weight in the aggregate of at least one member of thegroup consisting of boron carbide and titanium carbide and the remaindergraphite which additionally contains silicon carbide in a proportion of15 to 60 percent by weight of said graphite and said aggregate; saidcarbides being substantially uniformly distributed throughout saidarticle.

3. A formed oxidation-resistant article consisting essentially of 10 topercent by weight in the aggregate of at least one member of the groupconsisting of boron carbide and titanium carbide and the remaindergraphite 'which additionally contains silicon carbide in a proportion of30 to 50 percent by Weight of said graphite and said aggregate; saidcarbides being substantially uniformly distributed throughout saidarticle,

4. The method of making a formed oxidation-resistant article whichcomprises mixing at least one member of the group consisting of titaniumcarbide and boron carbide with carbon and a carbonizable binder; formingsaid mixture into at least one shaped article; baking said formedarticle, thereby carbonizing said binder and curing said article, in anon-oxidizing atmosphere; covering said baked article with asilicon-containing packing; and graphitizing said article at 2500" C. to2900 C.

5. The method of making a formed oxidation-resistant article whichcomprises mixing at least one member of the group consisting of titaniumcarbide and boron carbide with comminuted petroleum coke and coal tarpitch; forming said mixture into at least one shaped article; bakingsaid formed article to about 850 C., thereby carbonizing said pitch andcuring said article, in a protective coke covering; replacing at leastpart of said coke covering with sand; and graphitizing said article atabout 2700 C. to about 2800 C. for about 30 minutes.

References Cited in the file of this patent UNITED STATES PATENTS2,154,271 Higgins Apr. 11, 1939 2,307,550 Toepfer Ian. 5, 1943 2,358,615Arnott Sept. 14, 1944 2,906,632 Nickerson Sept. 29, 1959 2,908,553 FrankOct. 13, 1959 2,997,744 Stoddard et al Aug. 29, 1961 3,003,860 Sermon etal Oct. 10, 1961 3,019,128 Smiley Jan. 30, 1962

1. A FORMED GRAPHITE OXIDATION-RESISTANT ARTICLE HAVING SILICON CARBIDEAND AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF BORON CARBIDE ANDTITANIUM CARBIDE SUBSTANTIALLY UNIFORMLY DISTRIBUTED THERETHROUGH. 4.THE METHOD OF MAKING A FORMED OXIDATION-RESISTANT ARTICLE WHICHCOMPRISES MIXING AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF TITANIUMCARBIDE AND BORON CARBIDE WITH CARBON AND A CARBONIZABLE BINDER; FORMINGSAID MIXTURE INTO AT LEAST ONE SHAPED ARTICLE; BAKING SAID FORMEDARTICLE, THEREBY CARBONIZING SAID BINDER AND CURING SAID ARTICLE, IN ANON-OXIDIZING ATMOSPHERE; COVERING SAID BAKED ARTICLE WITH ASILICON-CONTAINING PACKING; AND GRAPHITIZING SAID ARTICLE AT 2500*C. TO2900*C.